1. Field of the Invention
This invention relates to a process of polymerizing or copolymerizing acrylonitrile in an aqueous medium in the presence of a redox catalyst to produce an acrylonitrile homopolymer or copolymer of improved whiteness and thermal stability.
A catalyst combination of an oxidizing agent and a reducing agent, i.e., a redox catalyst, is widely used for polymerizing in an aqueous medium acrylonitrile or copolymerizing acrylonitrile with another copolymerizable monoethylenically unsaturated compounds. However, the acrylonitrile polymers and copolymers prepared by the polymerization in an aqueous medium in the presence of a redox catalyst are not always satisfactory because they are frequently characterized by poor whiteness and poor thermal stability.
It has now been found that when the polymerization or copolymerization of acrylonitrile is performed in the presence of a specified redox catalyst, which comprises sulfurous acid, or its salts and nitrous acid or its salts, and of a water-insoluble mercaptan, polymers or copolymers of acrylonitrile of improved whiteness and thermal stability are obtainable with advantage.
2. Description of Prior Art
In general, mercaptans act as chain transfer agents in the polymerization, and therefore are widely used as molecular weight controlling agents to produce a polymer of desirably lower molecular weight. It is already known, as disclosed in Japanese Patent Publication No. 1619/1968, that the addition of a water-soluble mercaptan to the polymerization system involving a particular redox catalyst will yield a polymer of improved whiteness and thermal stability.
However, the addition of a water-soluble mercaptan is not advantageous, because this inevitably reduces conversion and increases the degree of polymerization as substantiated in the Comparative Examples below, and referred to in the Japanese Patent Publication No. 1619/1968. The reduction in conversion leads to a reduction in productivity of polymers, and an increase in the cost of production. In order to control the increase in the degree of polymerization, it is necessary to use a greater amount of the catalyst. Further, the reduction of the conversion and the increase in the degree of polymerization make it difficult to ensure a stable operation. If a water-soluble mercaptan is employed in a far less amount, in order to minimize the disadvantages mentioned above, polymers of improved whiteness and thermal stability are not obtainable.